Clad material

ABSTRACT

A clad material (30) includes a first layer (31) made of stainless steel, a second layer (32) made of Cu or a Cu alloy and roll-bonded to the first layer, and a third layer (33) made of stainless steel and roll-bonded to a side of the second layer opposite to the first layer. The clad material has an overall thickness of 1 mm or less, and in a cross-sectional view along a stacking direction, a minimum thickness of the first layer in the stacking direction and a minimum thickness of the third layer in the stacking direction are 70% or more and less than 100% of an average thickness of the first layer in the stacking direction and an average thickness of the third layer in the stacking direction, respectively.

TECHNICAL FIELD

The present invention relates to a clad material and a method formanufacturing the clad material.

BACKGROUND ART

In general, a clad material in which a first layer and a third layermade of stainless steel and a second layer made of Cu or a Cu alloyarranged between the first layer and the third layer are roll-bonded toeach other is disclosed in Japanese Patent Laid-Open No. 2005-219478,for example.

In a clad plate disclosed in Japanese Patent Laid-Open No. 2005-219478,covering layers (first and third layers) made of stainless steel arerespectively roll-bonded to both sides of a center layer (second layer)made of copper or a copper alloy, and the covering layers are bonded tothe center layer through barrier layers made of Nb or the like.Furthermore, the clad plate is roll-bonded by a thickness reductionprocess such as hot rolling or warm/cold rolling.

PRIOR ART DOCUMENT Patent Document

-   Patent Document 1: Japanese Patent Laid-Open No. 2005-219478

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

As a result of various studies, the inventor of the present inventionhas found that in the clad material disclosed in Japanese PatentLaid-Open No. 2005-219478, a portion having an excessively smallthickness may be generated in the first layer or the third layer made ofstainless steel, and thus when the clad material is welded to anothermember, the weld strength may decrease, and the mechanical strength ofthe clad material may vary.

The present invention has been proposed in order to solve theaforementioned problems, and an object of the present invention is toprovide a clad material and a method for manufacturing the clad materialcapable of significantly reducing or preventing a decrease in the weldstrength at the time of welding the clad material to another member anda variation in the mechanical strength of the clad material.

Means for Solving the Problems

As a result of earnest investigations in order to solve theaforementioned problems, the inventor of the present invention hasfurther found that due to the minimum thickness of the first layer inthe stacking direction and the minimum thickness of the third layer inthe stacking direction, respectively, being excessively smaller thantarget thicknesses (target values), the welding strength may decreasewhen the clad material is welded to another member, and a variation inthe mechanical strength of the clad material occurs. The presentinvention has been completed based on this finding. That is, a cladmaterial according to a first aspect of the present invention includes afirst layer made of stainless steel, a second layer made of Cu or a Cualloy and roll-bonded to the first layer, and a third layer made ofstainless steel and roll-bonded to a side of the second layer oppositeto the first layer and has an overall thickness of 1 mm or less, and ina cross-sectional view along a stacking direction, a minimum thicknessof the first layer in the stacking direction and a minimum thickness ofthe third layer in the stacking direction are 70% or more and less than100% of an average thickness of the first layer in the stackingdirection and an average thickness of the third layer in the stackingdirection, respectively.

Note that “stainless steel” refers to an alloy containing 50 mass % ormore of Fe (iron) as a main component and further containing at least10.5 mass % or more of Cr (chromium). Furthermore, “Cu alloy” refers toan alloy containing 50 mass % or more of Cu (copper) as a maincomponent. In addition, “the minimum thickness of the first layer in thestacking direction” and “the minimum thickness of the third layer in thestacking direction” respectively refer to the minimum value of thethickness of the first layer and the minimum value of the thickness ofthe third layer in a range of a predetermined length along a rollingdirection of the clad material in the cross-sectional view along thestacking direction.

In the clad material according to the first aspect of the presentinvention, as described above, in the cross-sectional view along thestacking direction, the minimum thickness of the first layer in thestacking direction and the minimum thickness of the third layer in thestacking direction are set to 70% or more and less than 100% of theaverage thickness of the first layer in the stacking direction and theaverage thickness of the third layer in the stacking direction,respectively. The clad material is configured in this manner such thatthe thicknesses of the first layer and the third layer in the stackingdirection are equalized, and thus the occurrence of portions havingexcessively small thicknesses (less than 70% of the average thicknesses)in the first layer and the third layer can be significantly reduced orprevented. Consequently, it is possible to significantly reduce orprevent a decrease in welding strength when the clad material is weldedto another member and to significantly reduce or prevent the occurrenceof a variation in the mechanical strength of the clad material.Therefore, when the clad material is welded to another member, forexample, welding of another member to the portion of the first layerhaving an excessively small thickness or the portion of the third layerhaving an excessively small thickness can be significantly reduced orprevented, and thus a decrease in welding strength due to insufficientwelding can be significantly reduced or prevented. In addition, avariation in the mechanical strength of the clad material can occur, andthus it is possible to significantly reduce or prevent the occurrence ofvariations in characteristics such as mechanical strength in a productproduced from the clad material. The advantageous effect of thisconfiguration is particularly effective when the overall thickness is assmall as 1 mm or less and the average thicknesses of the first layer andthe third layer are small (0.20 mm or less, for example). Furthermore,the occurrence of the portion having an excessively small thickness inthe first layer and the portion having an excessively small thickness inthe third layer is significantly reduced or prevented such that it ispossible to significantly reduce or prevent uncovering of the secondlayer due to tearing or pinholes occurring in the first layer and thethird layer at the time of rolling.

As described above, the clad material according to the first aspectincludes the first layer made of stainless steel, the second layer madeof Cu or a Cu alloy and roll-bonded to the first layer, and the thirdlayer made of stainless steel and roll-bonded to the side of the secondlayer opposite to the first layer. The clad material is configured inthis manner such that its mechanical strength and corrosion resistancecan be ensured by the first layer and the third layer made of stainlesssteel while its conductivity and thermal conductivity can be ensured bythe second layer made of Cu or a Cu alloy. Consequently, the cladmaterial suitable for a conductive member for batteries and a chassisthat also serves as a heat sink can be provided.

In the aforementioned clad material according to the first aspect, apercentage of a standard deviation of the minimum thickness of the firstlayer with respect to a thickness of the clad material and a percentageof a standard deviation of the minimum thickness of the third layer withrespect to the thickness of the clad material are preferably 1.5% orless. The clad material is configured in this manner such that theminimum thickness of the first layer in the stacking direction and theminimum thickness of the third layer in the stacking direction arefurther equalized, and thus the occurrence of the portion having anexcessively small thickness in the first layer and the portion having anexcessively small thickness in the third layer (the portions havingthicknesses of less than 70% of the average thicknesses) can be furthersignificantly reduced or prevented. Consequently, the occurrence of theportion having an excessively small thickness in the first layer made ofstainless steel and the portion having an excessively small thickness inthe third layer made of stainless steel can be further significantlyreduced or prevented. Therefore, the decrease in the welding strength asdescribed above can be further significantly reduced or prevented.Furthermore, the variations in the characteristics of the productproduced from the clad material as described above can be furthersignificantly reduced or prevented. The advantageous effect of thisconfiguration is particularly effective when the average thicknesses ofthe first layer and the third layer are small.

In the clad material according to the first aspect, the first layer andthe third layer are preferably both made of austenitic stainless steel.The clad material is configured in this manner such that the austeniticstainless steel and the Cu or Cu alloy are both non-magnetic such thatthe entire clad material can be non-magnetic. Accordingly, othercomponents (electronic components, for example) can be prevented frombeing adversely affected due to magnetization of the chassis when theclad material is used for the chassis that also serves as a heat sink,for example.

A method for manufacturing a clad material according to a second aspectof the present invention includes clad rolling for rolling and bonding afirst metal plate made of stainless steel, a second metal plate made ofCu or a Cu alloy, and a third metal plate made of stainless steel in astate in which the first metal plate, the second metal plate, and thethird metal plate are stacked in this order, and the clad rolling isperformed with a pressure-bonding load of 4.4×10³ N/m or more such thatthe clad material including a first layer made of stainless steel, asecond layer made of Cu or a Cu alloy and roll-bonded to the firstlayer, and a third layer made of stainless steel and roll-bonded to aside of the second layer opposite to the first layer, the clad materialhaving an overall thickness of 1 mm or less, in which in across-sectional view along a stacking direction, a minimum thickness ofthe first layer in the stacking direction and a minimum thickness of thethird layer in the stacking direction are 70% or more and less than 100%of an average thickness of the first layer in the stacking direction andan average thickness of the third layer in the stacking direction,respectively, is produced. Note that “a pressure-bonding load” in thepresent invention refers to a resultant force acting on a roller from arolled material (in the present invention, the first metal plate, andthe second metal plate, and the third metal plate) in clad rolling and aforce per unit length. This pressure-bonding load may be referred to asa rolling load or a roll force.

In the method for manufacturing a clad material according to the secondaspect of the present invention, as described above, the clad rolling isperformed with a pressure-bonding load of 4.4×10³ N/mm or more in astate in which the first metal plate made of stainless steel, the secondmetal plate made of Cu or a Cu alloy, and the third metal plate made ofstainless steel are stacked in this order. Thus, the clad rolling isperformed with a pressure-bonding load of 4.4×10³ N/mm or more, and thusit is possible to significantly reduce or prevent each layer from beingrolled in such a manner as to plastically deform non-uniformly due to adifference between the ductility of the first metal plate and the thirdmetal plate made of stainless steel and the ductility of the secondmetal plate made of Cu or a Cu alloy. Consequently, in the cladmaterial, the non-uniform thickness of the first layer in the stackingdirection and the non-uniform thickness of the third layer in thestacking direction can both be significantly reduced or prevented, andthus the occurrence of the portion having an excessively small thicknessin the first layer or the third layer made of stainless steel can besignificantly reduced or prevented. Furthermore, the occurrence of theportions having excessively small thicknesses in the first layer and thethird layer is significantly reduced or prevented such that it ispossible to significantly reduce or prevent uncovering of the secondlayer due to tearing or pinholes occurring in the first layer and thethird layer at the time of rolling.

In the aforementioned method for manufacturing a clad material accordingto the second aspect, the pressure-bonding load is preferably set to4.9×10³ N/mm or more. Accordingly, the clad rolling can be performedwith a more sufficient pressure-bonding load, and thus it is possible tofurther significantly reduce or prevent each layer from being rolled insuch a manner as to plastically deform non-uniformly.

In the aforementioned method for manufacturing a clad material accordingto the second aspect, it is preferably assumed that a prepared secondmetal plate has been work-hardened by being subjected to temper rollingafter annealing. Accordingly, as the second metal plate made of Cu or aCu alloy, a second metal plate, the mechanical strength (such as 0.2%proof stress) of which has been improved due to accumulation of theinternal stress (strain), can be used. Consequently, the mechanicalstrength of the second metal plate, which is lower in mechanicalstrength than the first metal plate and the third metal plate made ofstainless steel, can be closer to the mechanical strength of the fistmetal plate and the third metal plate. Therefore, the clad rolling canbe performed on the metal plates having similar mechanical strength, andthus each layer is rolled in such a manner as to plastically deform moreuniformly. Thus, the first metal plate, the second metal plate, and thethird metal plate can be sufficiently bonded to each other, and thethickness of each layer can be formed with a high degree of accuracy.

In this case, it is preferably assumed that a thickness of the preparedsecond metal plate after the temper rolling is 60% or more and less than100% of the thickness of the prepared second metal plate before thetemper rolling. Accordingly, excessive accumulation of internal stress(strain) in the prepared second metal plate due to the thickness of theprepared second metal plate after the temper rolling of less than 60% ofthe thickness of the prepared second metal plate before the temperrolling can be significantly reduced or prevented. Consequently, graincoarsening of the second layer (second metal plate) due to largeinternal stress (strain) can be significantly reduced or prevented, andthus a decrease in the elongation (workability) of the clad material dueto the second layer can be significantly reduced or prevented.

Effect of the Invention

According to the present invention, as described above, it is possibleto provide a clad material and a method for manufacturing the cladmaterial capable of significantly reducing or preventing a decrease inthe weld strength at the time of welding the clad material to anothermember and a variation in the mechanical strength of the clad material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 An exploded perspective view schematically showing a portabledevice using a clad material according to an embodiment of the presentinvention as a chassis.

FIG. 2 A cross-sectional view showing the structure of the clad material(chassis) according to the embodiment of the present invention.

FIG. 3 A schematic view for illustrating a clad material manufacturingprocess according to the embodiment of the present invention.

FIG. 4 A schematic view for illustrating the clad material manufacturingprocess according to the embodiment of the present invention.

FIG. 5 A front view showing a roller configured to produce the cladmaterial according to the embodiment of the present invention.

FIG. 6 An enlarged side view showing the roller configured to producethe clad material according to the embodiment of the present invention.

FIG. 7 A cross-sectional photograph of a pressure-bonded material undera pressure-bonding condition 3 in an experiment (first example)performed to confirm the effect of the present invention.

FIG. 8 A cross-sectional photograph of a pressure-bonded material undera pressure-bonding condition 13 in an experiment (first example)performed to confirm the effect of the present invention.

FIG. 9 A boxplot of an example of the present invention in an experiment(second example) performed to confirm the effect of the presentinvention.

FIG. 10 A boxplot of a comparative example in the experiment (secondexample) performed to confirm the effect of the present invention.

FIG. 11 A bar graph of the example of the present invention in theexperiment (second example) performed to confirm the effect of thepresent invention.

FIG. 12 A bar graph of the comparative example in the experiment (secondexample) performed to confirm the effect of the present invention.

MODES FOR CARRYING OUT THE INVENTION

An embodiment embodying the present invention is hereinafter describedon the basis of the drawings.

(Configuration of Portable Device)

A schematic configuration of a portable device 100 using a clad material30 as a chassis 3 according to an embodiment of the present invention isnow described with reference to FIGS. 1 and 2 .

As shown in FIG. 1 , the portable device 100 according to thisembodiment includes an upper housing 1 a, a display 2, the chassis 3, asubstrate 4, a battery 5, and a lower housing 1 b. The display 2, thechassis 3, the substrate 4, and the battery 5 are arranged in the lowerhousing 1 b in this order from above (Z1 side). The lower housing 1 b iscovered by the upper housing 1 a from above.

The display 2 is a liquid crystal display or an organic EL display, forexample, and has a function of displaying an image on the upper surfaceon the Z1 side.

The chassis 3 has a function of ensuring the mechanical strength of theportable device 100 and a function of releasing heat from the display 2,the substrate 4 (electronic components 4 a), and the battery 5 to theoutside. That is, the chassis 3 also serves as a heat sink. A member(not shown) of the portable device 100 is welded to the chassis 3.

The substrate 4 is arranged on the X1 side of the lower housing 1 b, andthe battery 5 is arranged on the X2 side. The electronic components 4 a,such as a central processing unit (CPU) configured to drive anapplication, are arranged on the upper surface of the substrate 4 on theZ1 side.

(Configuration of Chassis (Clad Material))

As shown in FIG. 2 , the chassis 3 is made of the three-layer cladmaterial 30 in which a SUS layer 31 made of stainless steel, a Cu layer32 made of Cu or a Cu alloy, and a SUS layer 33 made of stainless steelare stacked in this order. The Cu layer 32 is roll-bonded to a Z1 sidesurface (upper surface) of the SUS layer 33, and is roll-bonded to a Z2side surface (lower surface) of the SUS layer 31. At an interfacebetween the SUS layer 31 and the Cu layer 32 and at an interface betweenthe Cu layer 32 and the SUS layer 33, the layers are firmly bonded toeach other by forming an interatomic bond by diffusion annealing.Furthermore, a member (not shown) of the portable device 100 is weldedto the SUS layer 31 and the SUS layer 33. The SUS layer 31, the Cu layer32, and the SUS layer 33 are examples of a “first layer”, a “secondlayer”, and a “third layer” in the claims, respectively.

The thickness t1 of the clad material 30 in a Z direction is notparticularly limited. In this embodiment, the thickness t1 of thechassis 3 is preferably 1 mm or less, and more preferably 0.5 mm or lessin order to significantly reduce or prevent an increase in the thicknessin the Z direction in consideration of weight reduction and compactnessof the portable device 100. Furthermore, in order to ensure themechanical strength of the chassis 3 and to significantly reduce orprevent difficulty in manufacturing, the thickness t1 of the chassis 3is preferably 0.03 mm or more, more preferably 0.05 mm or more, andstill more preferably 0.1 mm or more. From the viewpoint of heatdissipation, the Cu layer 32 is preferably thicker than the SUS layers(31, 33). However, when the thickness of the Cu layer 32 is too large,the thicknesses of the SUS layers (31, 33) become small, and tearing orpinholes may occur when the Cu layer 32 is roll-bonded. Consequently, aportion of the Cu layer 32 not covered with the SUS layers (31, 33) maybe generated. Therefore, the layer thickness ratio (SUS layer:Culayer:SUS layer) is preferably such that when the thickness of the SUSlayer is 1, the thickness of the Cu layer is 2 or more and 8 or less(1:2:1 to 1:8:1).

The stainless steel making up the SUS layer 31 and the SUS layer 33 isnot particularly limited as long as the same is stainless steel, such asaustenitic, ferritic, and martensitic stainless steel. In thisembodiment, it is not preferable to magnetize the chassis 3 in theportable device 100 including the electronic components 4 a (see FIG. 1). Therefore, the stainless steel making up the SUS layer 31 and the SUSlayer 33 is preferably austenitic stainless steel, and more preferablyso-called SUS300 (JIS standards) austenitic stainless steel.

Furthermore, the stainless steel making up the SUS layer 31 and the SUSlayer 33 is particularly preferably SUS316L (JIS standards), which has alow C (carbon) content and is less magnetic, of the austenitic stainlesssteel. Note that SUS316L is austenitic stainless steel with a reduced Ccontent of SUS316 (JIS standards) containing 18 mass % of Cr, 12 mass %of Ni, 2.5 mass % of Mo, inevitable impurities including C, and thebalance Fe (iron). The SUS layer 31 and the SUS layer 33 are not limitedto the same composition, but are preferably made of stainless steelhaving the same composition in consideration of rolling stability etc.

The Cu layer 32 is made of Cu of C1000 series (JIS standards) or a Cualloy such as C2000 series (JIS standards). As the copper, there areso-called oxygen-free copper, phosphorus deoxidized copper, tough pitchcopper, etc. As the Cu alloy, a Zr—Cu alloy of C1050 (JIS standards),for example, is preferable in order to significantly reduce or preventgrain coarsening. The Cu or Cu alloy making up the Cu layer 32 generallyhas higher thermal conductivity and greater ductility than the stainlesssteel making up the SUS layer 31 and the SUS layer 33. Furthermore, thegrain size of the Cu layer 32 (Cu or Cu alloy) measured by a comparisonmethod of JIS H 0501 is preferably 0.150 mm or less, and a decrease inthe elongation (ductility) of the Cu layer 32 can be significantlyreduced or prevented.

In the clad material 30 according to this embodiment, the SUS layers 31and 33 have variations in the thicknesses t2 and t3 along a rollingdirection, respectively. Consequently, the SUS layers 31 and 33 haveportions having minimum thicknesses t2min and t3min smaller than thethicknesses t2 and t3 of the other portions, respectively. On the otherhand, the thickness t4 of the Cu layer 32 hardly varies. Consequently,when the thicknesses t2 and t3 vary, the interfaces of the clad material30 become wavy in a cross-sectional view along a stacking direction (Zdirection). In FIG. 2 , the wave shapes of the interfaces areexaggerated.

In this embodiment, in the clad material 30, the minimum thickness t2minof the SUS layer 31 in the stacking direction and the minimum thicknesst3min of the SUS layer 33 in the stacking direction are 70% or more andless than 100% of the average thickness t2avg of the SUS layer 31 in thestacking direction and the average thickness t3avg of the SUS layer 33in the stacking direction in the cross-sectional view along the stackingdirection (Z direction), respectively.

The minimum thickness t2min of the SUS layer 31 in the stackingdirection is the minimum thickness t2 of the SUS layer 31 in apredetermined range (length) in the rolling direction of the cladmaterial 30. Similarly, the minimum thickness t3min of the SUS layer 33in the stacking direction is the minimum thickness t3 of the SUS layer33 in a predetermined range (length) in the rolling direction of theclad material 30. The predetermined range (length) of the clad material30 is not particularly limited as long as the same is in a range of oneor more wavelengths of the wave of the interface formed in a wave shape,but the same is preferably at least about 15 mm from the viewpoint ofmeasurement reliability. Furthermore, the average thickness t2avg of theSUS layer 31 in the stacking direction is the average of the thicknesst2 of the SUS layer 31 in the clad material 30. Similarly, the averagethickness t3avg of the SUS layer 33 in the stacking direction is theaverage of the thickness t3 of the SUS layer 33 in the clad material 30.Note that it is better to obtain the average thicknesses t2avg and t3avgby randomly measuring the thicknesses t2 and t3 at a plurality ofportions (ten or more portions, for example) of the SUS layers 31 and 33in the predetermined range (length) described above, for example, of theclad material 30 and calculating the averages of the plurality ofmeasured thicknesses t2 and t3. On the other hand, it is better toobtain the thickness t4 of the Cu layer 32 as the average thicknesst4avg (there is almost no variation, and thus it is hereinafter simplyreferred to as “t4”) in the same manner as the average thicknesses t2avgand t3avg of the SUS layers 31 and 33 described above.

The percentage (%) of the standard deviation of the minimum thicknesst2min of the SUS layer 31 with respect to the thickness t1 of the cladmaterial 30 and the percentage (%) of the standard deviation of theminimum thickness t3min of the SUS layer 33 with respect to thethickness t1 of the clad material 30 are both preferably 1.5% or less,and more preferably 1.2% or less. When the standard deviation isobtained, it is preferable to acquire the minimum thicknesses t2min andt3min of more portions (one hundred or more portions, for example).

When the composition (material) of the stainless steel making up the SUSlayer 31 and the composition (material) of the stainless steel making upthe SUS layer 33 are the same while the thickness ratios (t2avg/t1,t3avg/t1) of the average thicknesses (t2avg, t3avg) to the thickness(t1) of the clad material 30 are the same, the minimum thicknesses andvariations in the minimum thicknesses in the SUS layer 31 and the SUSlayer 33 are conceivably equal. Therefore, in this case, data of the SUSlayer 31 and data of the SUS layer 33 can be combined and evaluated tobe treated as the minimum thickness and the standard deviation of a pairof SUS layers.

The ratio (t2avg:t4:t3avg) of the average thickness t2avg of the SUSlayer 31, the thickness t4 of the Cu layer 32, and the average thicknesst3avg of the SUS layer 33 in the clad material 30 is not particularlylimited. Note that in order to equalize the degree of elongation in cladrolling or the like on both sides in the Z direction, the averagethickness t2avg of the SUS layer 31 and the average thickness t3avg ofthe SUS layer 33, both of which are made of stainless steel, arepreferably substantially equal.

It is preferable to vary the thickness ratio according to thecharacteristics (thermal conductivity and mechanical strength) requiredfor the chassis 3. For example, when the mechanical strength isparticularly required in the chassis 3, it is preferable to increase theaverage thicknesses t2avg and t3avg of the SUS layers 31 and 33 made ofstainless steel having high mechanical strength. In order to ensure themechanical strength of the clad material 30, the thickness t4 of the Culayer 32 is preferably 60% or less of the thickness t1 of the cladmaterial 30. On the other hand, when the thermal conductivity isparticularly required for the chassis 3, it is preferable to increasethe thickness t4 of the Cu layer 32. In order to ensure the thermalconductivity of the clad material 30, the thickness t4 of the Cu layer32 is preferably 33% or more of the thickness t1 of the clad material30. Furthermore, the elongation of the clad material 30 is preferably 8%or more, and more preferably 10% or more from the viewpoint of pressworkability, for example.

(Outline of Method for Manufacturing Chassis (Clad Material))

A method for manufacturing the clad material 30 making up the chassis 3according to the embodiment of the present invention is now describedwith reference to FIGS. 2 to 6 .

First, as shown in FIG. 3 , a strip-shaped Cu plate 132 made of Cu or aCu alloy is prepared. Then, soft annealing is performed on the Cu plate132 using an annealing furnace 101 in which the internal temperature isset to a temperature exceeding the recrystallization temperature (220°C., for example) of the Cu or Cu alloy making up the Cu plate 132. Thus,the Cu plate 132 is in a state in which the internal strain due towork-hardening is removed and the structure is sufficiently softened.

Then, temper rolling is performed on the Cu plate 132 that has undergonethe soft annealing using a roller 102. By the temper rolling, the Cuplate 132 accumulates internal stress (strain) and is work-hardened.Furthermore, the number of temper rolling passes can be selected asappropriate.

In the temper rolling, rolling is preferably performed in such a mannerthat the thickness t14 of the Cu plate 132 after the temper rolling is60% or more and less than 100% of the thickness t14 of the Cu plate 132before the temper rolling. Thus, during clad rolling described below,the ductility of SUS plates 131 and 133 made of stainless steel havinghigh MECHANICAL STRENGTH and low ductility and the ductility of a Cuplate 132 made of Cu or a Cu alloy having low mechanical strength andhigh ductility can be close to each other.

The thickness t14 of the Cu plate 132 after the temper rolling is set to60% or more of the thickness t14 of the Cu plate 132 before the temperrolling such that grain coarsening of the Cu layer 32 of the cladmaterial 30 can be easily significantly reduced or prevented. Thus, itcan be assumed that the prepared Cu plate 132 has been work-hardened bybeing subjected to the temper rolling after the annealing.

As shown in FIG. 4 , in addition to the strip-shaped Cu plate 132 thathas undergone the temper rolling after the annealing, the strip-shapedSUS plate 131 and the strip-shaped SUS plate 133 made of stainless steelare prepared. Furthermore, both the SUS plates 131 and 133 aresufficiently annealed. The thickness of the SUS plate 131, the thicknessof the Cu plate 132 that has undergone the temper rolling, and thethickness of the SUS plate 133 are appropriately selected according tothe thickness ratios (t2avg:t4:t3avg) of the SUS layer 31, the Cu layer32, and the SUS layer 33 in the clad material 30 to be produced.

Then, the clad rolling is performed to roll and bond the sufficientlyannealed SUS plate 131, the Cu plate 132 that has undergone the temperrolling after the annealing, and the sufficiently annealed SUS plate 133using a roller 103 in a state in which the same are stacked in thisorder. Thus, a pressure-bonded material 130 having a thickness of 1.0 mmor less or 0.5 mm or less, for example, in which the SUS plate 131, theCu plate 132, and the SUS plate 133 have been bonded (roll-bonded) toeach other in a state in which the same are stacked in this order, isproduced. The number of clad rolling passes can be selected asappropriate.

As shown in FIG. 5 , the roller 103 includes a pair of work rollers 103a and a pair of work rollers 103 b, four bearings 103 c thatrespectively hold shafts of the work rollers 103 a and 103 b, and a pairof load cells 103 d attached to the bearings 103 c of the work rollers103 a on the Z1 side, for example. The pair of load cells 103 d have afunction of detecting a force (resultant force) P0 (N) that acts on thework rollers 103 a from the SUS plate 131, the Cu plate 132, and the SUSplate 133 (rolled material) by detecting the strain caused by the force(resultant force) that acts on the work rollers 103 a.

When the clad rolling is performed, it is important to first determine apressure-bonding load. As described in “Plate Rolling” (edited by theJapan Society for Technology of Plasticity, Corona Publishing Co., Ltd.,Feb. 15, 1993, First Edition), plate thickness control differs dependingon a rolling mill and a rolling method used. Therefore, it is necessaryto consider a rolling mill and a rolling method in order to obtain adesired plate thickness. On the other hand, the thickness of thepressure-bonded material on the exit side of the roller can becalculated from values such as a pressure-bonding load, a plate width, aroll force function, and deformation resistance. Therefore, thepressure-bonding load is determined such that it is possible to setvarious conditions for obtaining a desired plate thickness according tothe used rolling mill. Furthermore, the plate thickness is equal to thesum of the deformation of the rolling mill due to the pressure-bondingload and the roll setting position of the rolling mill. Therefore, inorder to obtain a desired plate thickness, the plate thickness can beadjusted by changing the roll setting position.

In addition, as described in “Plate Rolling” (Corona Publishing Co.,Ltd., first published on Oct. 25, 1960), a rational rolling plan isassociated with a pressure-bonding load. The pressure-bonding load canbe calculated from various conditions such as a thickness before rollingand a roll radius, and thus a rational rolling plan can be made withoutprescribing other conditions by determining the pressure-bonding load inadvance.

In the manufacturing method according to this embodiment, apressure-bonding load P in the clad rolling is set to 4.4×10³ N/mm ormore. When the length of a roller surface in the axial direction of thework rollers 103 a (103 b) (also referred to as a “width direction” in adirection orthogonal to the rolling direction and the stackingdirection) is L (mm), the pressure-bonding load P can be obtained fromthe following formula (1) using the force P0 applied to the work rollers103 a.P=P0/L  (1)

The pressure-bonding load P (N/mm) can be obtained as an estimate fromthe following formula (2).P=Qp·k·√{square root over ( )}(R·(h1−h2))/L  (2)

In the above formula (2), the roll force function (magnificationcompared with ideal deformation) is Qp, and the average deformationresistance (an average stress required for deformation between therollers in a two-dimensional deformation state in the case in which thedeformation in the width direction is ignored) is k (N/mm²). Referringto FIG. 6 , the radii of the work rollers 103 a (103 b) are R (mm), thetotal thickness of the SUS plate 131, the Cu plate 132, and the SUSplate 133 (rolled material) on the entrance side of the roller 103 is h1(mm), and the thickness of the pressure-bonded material 130 on the exitside of the roller 103 is h2 (mm).

It is possible to increase the pressure-bonding load P by appropriatelycombining increasing the radii of the work rollers 103 a (103 b),increasing the radii of the work rollers 103 a (103 b), increasing acoefficient of dynamic friction between the work rollers 103 a (103 b)and the rolled material, increasing a rolling reduction (=(h1−h2)/h1) inthe clad rolling, reducing a force (tension) along the rolling directionapplied to the rolled material, reducing the transport speed of therolled material, etc. When the clad rolling is performed continuously,it is possible to adjust the pressure-bonding load P while continuouslyperforming the clad rolling by adjusting a pressure-bonding speed(transport speed) or tension.

In order to significantly reduce or prevent the occurrence of a portionof the SUS layer 31 having an excessively small thickness t2 and aportion of the SUS layer 33 having an excessively small thickness t3 inthe clad material 30 (see FIG. 2 ) as a final product, thepressure-bonding load P is preferably 4.9×10³ N/mm or more, morepreferably 6.0×10³ N/mm or more, and still more preferably and 6.8×10³N/mm or more. On the other hand, when the clad rolling is continuouslyperformed, a low pressure-bonding speed is not preferable because thetact time of the clad material 30 increases. Therefore, when priority isgiven to reducing the tact time, it is preferable to set thepressure-bonding load P to 4.4×10³ N/mm or more and a value in thevicinity of 4.4×10³ N/mm.

As shown in FIG. 4 , in the pressure-bonded material 130 immediatelyafter the clad rolling, the thickness t12 of the SUS plate 131 and thethickness t13 of the SUS plate 133 are varied along the rollingdirection. Consequently, there are portions of the SUS plates 131 and133 respectively having a minimum thickness t12min and a minimumthickness t13min due to a partial difference in thickness.

Furthermore, in the manufacturing method according to this embodiment,the pressure-bonding load P in the clad rolling is set to 4.4×10³ N/mmor more. Thus, in the cross-sectional view along the stacking direction(Z direction) of the pressure-bonded material 130, the minimum thicknesst12min of the SUS plate 131 in the stacking direction and the minimumthickness t13min of the SUS plate 133 in the stacking direction can beset to 84% or more and less than 100% of the average thickness t12avg ofthe SUS plate 131 in the stacking direction and the average thicknesst13avg of the SUS plate 133 in the stacking direction, respectively.

Thus, even when the variations in the thickness t12 of the SUS plate 131and the thickness t13 of the SUS plate 133 along the rolling directionare further increased by performing further rolling (intermediaterolling and finish rolling described below) after the clad rolling. Inthe clad material 30 as the final product, the minimum thickness t2minof the SUS layer 31 and the minimum thickness t3min of the SUS layer 33can be 70% or more and less than 100% of the average thickness t2avg andthe average thickness t3avg, respectively.

The average thicknesses t12avg and t13avg of the SUS plates 131 and 133may be obtained from a predetermined range (length) in the rollingdirection of the clad material. The predetermined range (length) is notparticularly limited as long as the same is in the range of one or morewavelengths of the wave of the interface formed in a wave shape, but ispreferably at least about 15 mm from the viewpoint of measurementreliability. Furthermore, the average thickness t12avg of the SUS plate131 is the average of the thickness t12 of the SUS plate 131 in thepressure-bonded material 130, and the average thickness t13avg of theSUS plate 133 is the average of the thickness t13 of the SUS plate 133in the pressure-bonded material 130.

Thereafter, if necessary, the length of the pressure-bonded material 130in the width direction may be adjusted by cutting an end of thepressure-bonded material 130 in the width direction using an end cuttingmachine 104. Then, the thickness of the pressure-bonded material 130 isadjusted by performing the intermediate rolling on the pressure-bondedmaterial 130 using a roller 105. Thus, it is possible to reducethickness variations in each pressure-bonded material 130 (clad material30). The number of passes for intermediate rolling can be selected asappropriate.

Then, diffusion annealing is performed using an annealing furnace 106 inwhich the internal temperature is set to a temperature exceeding therecrystallization temperature of the stainless steel making up the SUSplate 131. At this time, in order to significantly reduce or preventcoarsening of the grain size of the Cu plate 132 resulting from theannealing, it is preferable to perform the diffusion annealing under atemperature condition of 850° C. or more and 1000° C. or less. Thus, allthe SUS plate 131, the Cu plate 132, and the SUS plate 131 are in astate in which the structure has been softened according to thematerial. At the interface between the SUS plate 131 and the Cu plate132 and at the interface between the Cu plate 132 and the SUS plate 133,the layers form an interatomic bond and are firmly bonded by diffusiontreatment.

Thereafter, finish rolling is performed in order to adjust the thicknessof the pressure-bonded material 130 after the diffusion annealing.Consequently, the clad material 30 including the SUS layer 31, the Culayer 32 roll-bonded to the SUS layer 31, and the SUS layer 33roll-bonded to the side of the Cu layer 32 opposite to the SUS layer 31shown in FIG. 2 is produced.

Thereafter, shape correction, slit cutting, press (punching) work, etc.are appropriately performed on the clad material 30 as necessary.Consequently, the chassis 3 made of the clad material 30 shown in FIG. 2is produced.

In this embodiment, as shown in FIG. 4 , at least the steps from theclad rolling to the slit cutting in a finishing step are continuouslyperformed, and thus the tact time of the clad material 30 can beeffectively reduced. The manufacturing method according to the presentinvention is not limited to a method in which the steps from the cladrolling to the slit cutting in the finishing step are continuouslyperformed.

When rolling (general cold rolling) is performed on the press-bondedmaterial 130 after the clad rolling, the average thickness of each layeris reduced in accordance with the rolling reduction, but the ratio ofthe average thickness of each layer is substantially equivalent andunchanged. Based on this viewpoint, the aforementioned average thicknesst2avg of the SUS layer 31 of the clad material 30(=t1×t12/(t12+t13+t14)) may be acquired by multiplying the thickness t1(see FIG. 2 ) of the clad material 30 by the ratio (=t12/(t12+t13+t14))of the thickness t12 of the prepared SUS plate 131 to the totalthickness (t12+t13+t14) of the prepared SUS plate 131, Cu plate 132, andSUS plate 133. Similarly, the aforementioned average thickness t3avg ofthe SUS layer 33 of the clad material 30 (=t1×t13/(t12+t13+t14)) may beacquired by multiplying the thickness t1 of the clad material 30 by theratio (=t13/(t12+t13+t14)) of the thickness t13 of the prepared SUSplate 133 to the total thickness of the prepared SUS plate 131, Cu plate132, and SUS plate 133.

Furthermore, the aforementioned average thickness t12avg of the SUSplate 131 of the pressure-bonded 130 (=t11×t12/(t12+t13+t14)) may beacquired by multiplying the thickness t11 of the pressure-bondedmaterial 130 by the ratio (=t12/(t12+t13+t14)) of the thickness t12 ofthe prepared SUS plate 131 to the total thickness (=t12+t13+t14) of theprepared SUS plate 131, Cu plate 132, and SUS plate 133. Similarly, theaforementioned average thickness t13avg of the SUS plate 133 of thepressure-bonded material 130 (=t11×t13/(t12+t13+t14)) may be acquired bymultiplying the thickness t11 of the pressure-bonded material 130 by theratio (=t13/(t12+t13+t14)) of the thickness t13 of the prepared SUSplate 133 to the total thickness (=t12+t13+t14) of the prepared SUSplate 131, Cu plate 132, and SUS plate 133.

Note that the average thicknesses t12avg and t13avg may be obtained bymeasuring the thicknesses t2, t3, t12, and t13 at a plurality ofportions (ten or more portions, for example) of the corresponding layersand plates and calculating the averages thereof.

In the manufacturing method according to this embodiment, the thicknesst14 (see FIG. 4 ) of the Cu plate 132 before the diffusion annealing(after the intermediate rolling) is preferably set to 20% or more of thethickness t14 (see FIG. 3 ) of the Cu plate 132 after the soft annealing(before the temper rolling). Thus, the grain size of the Cu or Cu alloyin the Cu layer 32 (the Cu plate 132 after the diffusion annealing) canbe reduced to 150 μm or less. Consequently, the elongation of the cladmaterial 30 is improved, and thus workability can be improved.

Advantageous Effects of this Embodiment

In this embodiment, the following advantageous effects are achieved.

In this embodiment, as described above, in the cross-sectional viewalong the stacking direction (Z direction), the minimum thickness t2minof the SUS layer 31 in the stacking direction and the minimum thicknesst3min of the SUS layer 33 in the stacking direction are set to 70% ormore and less than 100% of the average thickness t2avg of the SUS layer31 in the stacking direction and the average thickness t3avg of the SUSlayer 33 in the stacking direction, respectively. Accordingly, in theclad material 30, the thicknesses t2 and t3 of the SUS layer 31 and theSUS layer 33 in the stacking direction are equalized, and thus theoccurrence of portions having excessively small thicknesses (less than70% of the average thicknesses) in the SUS layer 31 and the SUS layer 33can be significantly reduced or prevented. Therefore, when the chassis 3making up the clad material 30 is welded to another member, for example,welding of another member to the portion of the SUS layer 31 having anexcessively small thickness t2 or the portion of the SUS layer 33 havingan excessively small thickness t3 can be significantly reduced orprevented, and thus a decrease in welding strength due to insufficientwelding can be significantly reduced or prevented. In addition, it ispossible to significantly reduce or prevent the occurrence of variationsin characteristics such as mechanical strength in the product (chassis3) produced from the clad material 30. Furthermore, the occurrence ofthe portion having an excessively small thickness in the SUS layer 31and the portion having an excessively small thickness in the SUS layer33 is significantly reduced or prevented such that it is possible tosignificantly reduce or prevent uncovering of the Cu layer 32 due totearing or pinholes occurring in the SUS layer 31 and the SUS layer 33at the time of rolling.

In this embodiment, as described above, the clad material 30 includesthe SUS layer 31 made of stainless steel, the Cu layer 32 made of Cu ora Cu alloy and roll-bonded to the SUS layer 31, and the SUS layer 33made of stainless steel and roll-bonded to the side of the Cu layer 32opposite to the SUS layer 31. Accordingly, in the clad material 30, itsmechanical strength and corrosion resistance can be ensured by the SUSlayer 31 and the SUS layer 33 made of stainless steel while itsconductivity and thermal conductivity can be ensured by the Cu layer 32made of Cu or a Cu alloy. Consequently, the clad material 30 suitablefor the chassis 3 that also serves as a heat sink can be provided.

In this embodiment, the percentage of the standard deviation of theminimum thickness t2min of the SUS layer 31 with respect to thethickness t1 of the clad material 30 and the percentage of the standarddeviation of the minimum thickness t3min of the SUS layer 33 withrespect to the thickness t1 of the clad material 30 are preferably 1.5%or less. Accordingly, the minimum thickness t2min of the SUS layer 31 inthe stacking direction and the minimum thickness t3min of the SUS layer33 in the stacking direction are further equalized, and thus theoccurrence of the portion having an excessively small thickness in theSUS layer 31 and the portion having an excessively small thickness inthe SUS layer 33 (the portions having thicknesses of less than 70% ofthe average thicknesses) can be further significantly reduced orprevented. Consequently, the occurrence of the portion having anexcessively small thickness in the SUS layer 31 and the portion havingan excessively small thickness in the SUS layer 33 can be furthersignificantly reduced or prevented. Therefore, the decrease in thewelding strength as described above can be further significantly reducedor prevented. Furthermore, the variations in the characteristics of thechassis 3 produced from the clad material 30 as described above can befurther significantly reduced or prevented.

In this embodiment, the SUS layer 31 and the SUS layer 33 are preferablyboth made of austenitic stainless steel. Accordingly, the austeniticstainless steel and the Cu or Cu alloy are both non-magnetic such thatthe entire clad material 30 can be non-magnetic. Accordingly, theelectronic components 4 a, for example, can be prevented from beingadversely affected due to magnetization of the chassis 3 made of theclad material 30 and also serving as a heat sink.

In the manufacturing method according to this embodiment, in a state inwhich the SUS plate 131 made of stainless steel, the Cu plate 132 madeof Cu or a Cu alloy, and the SUS plate 133 made of stainless steel arestacked in this order, the clad rolling is performed with apressure-bonding load P of 4.4×10³ N/mm or more. Accordingly, the cladrolling is performed with a sufficient pressure-bonding load P of4.4×10³ N/mm or more, and thus it is possible to significantly reduce orprevent each layer from being rolled in such a manner as to plasticallydeform non-uniformly due to a difference between the ductility of theSUS plate 131 and the SUS plate 133 made of stainless steel and theductility of the Cu plate 132 made of Cu or a Cu alloy. Consequently, inthe clad material 30, the non-uniform thickness t2 of the SUS layer 31in the stacking direction and the non-uniform thickness t3 of the SUSlayer 33 in the stacking direction can both be significantly reduced orprevented, and thus in the cross-sectional view along the stackingdirection (Z direction), the minimum thickness t2min of the SUS layer 31in the stacking direction and the minimum thickness t3min of the SUSlayer 33 in the stacking direction can be set to 70% or more and lessthan 100% of the average thickness t2avg of the SUS layer 31 in thestacking direction and the average thickness avg of the SUS layer 33 inthe stacking direction, respectively. Therefore, the occurrence of theportion having an excessively small thickness in the SUS layer 31 or theSUS layer 33 made of stainless steel can be significantly reduced orprevented.

In the manufacturing method according to this embodiment, thepressure-bonding load P is preferably set to 4.9×10³ N/mm or more.Accordingly, the clad rolling can be performed with a more sufficientpressure-bonding load P, and thus it is possible to furthersignificantly reduce or prevent each layer from being rolled in such amanner as to plastically deform non-uniformly.

In the manufacturing method according to this embodiment, it is assumedthat the prepared Cu plate 132 has been work-hardened by being subjectedto the temper rolling after the annealing. Accordingly, as the Cu plate132 made of Cu or a Cu alloy, a Cu plate 132, the mechanical strength(such as 0.2% proof stress) of which has been improved due toaccumulation of the internal stress (strain), can be used. Consequently,the mechanical strength of the Cu plate 132, which is lower inmechanical strength than the SUS plate 131 and the SUS plate 133 made ofstainless steel, can be closer to the mechanical strength of the SUSplate 131 and the SUS plate 133. Therefore, the clad rolling can beperformed on the metal plates having similar mechanical strength, andthus each layer is rolled in such a manner as to plastically deform moreuniformly. Thus, the SUS plate 131, the Cu plate 132, and the SUS plate133 can be sufficiently bonded to each other, and the thickness of eachlayer can be formed with a high degree of accuracy.

In the manufacturing method according to this embodiment, it ispreferably assumed that the thickness of the prepared Cu plate 132 afterthe temper rolling is 60% or more and less than 100% of the thicknessbefore the temper rolling. Accordingly, excessive accumulation ofinternal stress (strain) in the prepared Cu plate 132 due to thethickness of the prepared Cu plate 132 after the temper rolling of lessthan 60% of the thickness of the prepared Cu plate 132 before the temperrolling can be significantly reduced or prevented. Consequently, graincoarsening of the Cu layer 32 (Cu plate 132) due to large internalstress (strain) can be significantly reduced or prevented, and thus adecrease in the elongation (workability) of the clad material 30 due tothe Cu layer 32 can be significantly reduced or prevented.

EXAMPLES

First to third examples performed to confirm the effect of the presentinvention are now described with reference to FIGS. 3, 4, and 7 to 12 .

First Example

As the first example, in press-bonded materials each including a pair ofa SUS plate (first metal plate) and a SUS plate (third metal plate)immediately after clad rolling, the thicknesses (minimum thicknessestαmin) of portions of the SUS plates having the smallest thickness(thinnest portions) and the average values (average thicknesses tαavg)of the thicknesses of the SUS plates were acquired when the clad rollingwas performed with different pressure-bonding loads P.

Specifically, pressure-bonded materials were produced by themanufacturing method shown in FIGS. 3 and 4 . First, a Cu plate 132(second metal plate) made of oxygen-free copper (C1020, JIS standards)and having a thickness t14 of 0.5 mm was prepared. This Cu plate had along strip shape in a rolling direction. Then, as shown in FIG. 3 , softannealing was performed on the Cu plate 132 at a temperature higher thanthe recrystallization temperature of Cu making up the Cu plate 132, andthen temper rolling was performed. Thus, the thickness t14 of the Cuplate 132 after the temper rolling was set to 0.4 mm (80% of thethickness t14 before the temper rolling), and the Cu plate waswork-hardened to some extent.

A pair of SUS plates 131 and 133 (a first metal plate and a third metalplate) having a thickness of 0.2 mm, made of SUS316L (JIS standards),and commonly frequently used, were prepared. As the pair of SUS plates131 and 133, sufficiently annealed SUS plates were used. Furthermore,the pair of SUS plates 131 and 133 had a long strip shape in the rollingdirection.

The thickness ratio (t12:t14:t13) of the thickness t12 (=0.2 mm) of theSUS plate 131, the thickness t14 (=0.4 mm) of the Cu plate 132, and thethickness t13 (=0.2 mm) of the SUS plate 133 was 1:2:1. Preparedmaterials for the SUS plate 131 and the SUS plate 133 and thethicknesses thereof were the same, and thus the thickness ratios of thethickness (average thickness t12avg) of the SUS plate 131 and thethickness (average thickness t13avg) of the SUS plate 133 to thethickness of the pressure-bonded material 130 were substantially thesame (25%). In this case, the SUS plate 131 and the SUS plate 133 can becollectively evaluated as SUS plates (the SUS plates 131 and 133)without distinguishing the SUS plate 131 from the SUS plate 133 in thepressure-bonded material 130.

Then, the clad rolling was performed in a state in which the SUS plate131, the work-hardened Cu plate 132, and the SUS plate 133 were stackedin this order such that the strip-shaped pressure-bonded material 130was produced. At this time, the rolling was performed such that thethickness t12 of the SUS plate 131, the thickness t14 of the Cu plate132, and the thickness t13 of the SUS plate 133 after the clad rollingbecame 56% of the thickness t12, the thickness t14, and the thicknesst13 before the clad rolling.

In the clad rolling, the SUS plate 131, the work-hardened Cu plate 132,and the SUS plate 133 were roll-bonded under any one of pressure-bondingconditions 1 to 13 shown in TABLE 1. Then, a cross-section (see FIG. 4 )of the pressure-bonded material 130 immediately after the clad rollingin the stacking direction (Z direction), which was a cross-section alongthe rolling direction, was observed such that the thickness (minimumthickness t12min) of the thinnest portion of the SUS plate 131 and thethickness (minimum thickness t13min) of the thinnest portion of the SUSplate 133 were acquired.

Specifically, in the strip-shaped pressure-bonded material 130 producedwith any one of thirteen types of pressure-bonding loads P (4.0×10³ N/mm(pressure-bonding condition 1) or more and 7.8×10³ N/mm(pressure-bonding condition 13) or less) shown in TABLE 1, ten testregions having a range (length) of 15 mm in the rolling direction wereacquired. At this time, five portions were randomly acquired in thevicinity of each of opposite ends of the strip-shaped pressure-bondedmaterial 130 in the rolling direction. Then, in each of the ten testregions, the minimum thickness t12min of the SUS plate 131 and theminimum thickness t13min of the SUS plate 133 were acquired, the tenminimum thicknesses t12min and the ten minimum thicknesses t13min werecollectively averaged, and the minimum thickness of the SUS plates (SUSplates 131 and 133) in the pressure-bonded material 130 was set totαmin. Therefore, the above minimum thickness tαmin is based on measuredvalues of twenty thicknesses obtained from the pressure-bonded material130.

For each of the ten test regions, the thicknesses of five randomlyselected portions of the SUS plate 131 was measured and averaged toacquire the average thickness t12avg of the SUS plate 131. Similarly,for each of the ten test regions, the thicknesses of five randomlyselected portions of the SUS plate 133 were measured and averaged toacquire the average thickness t13avg of the SUS plate 133. Then, theaverage thickness t12avg of the five portions and the average thicknesst13avg of the five portions obtained from the ten test regions werecollectively averaged to obtain the average thickness tαavg of the SUSplates (SUS plates 131 and 133) of the pressure-bonded material 130.Therefore, the above average thickness tαavg is based on measured valuesof a hundred thicknesses obtained from the pressure-bonded material 130.

Next, the thickness ratio Rα1 (=(tαmin/t11)×100(%)) of the above minimumthickness tαmin of the SUS plates (SUS plates 131 and 133) to thethickness t11 of the entire pressure-bonded material 130 was calculated.Furthermore, the thickness ratio Rα2 (=(tαmin/tαavg)×100(%)) of theminimum thickness tαmin to the above average thickness tαavg of the SUSplates (SUS plates 131 and 133) with respect to the thickness t11 of theentire pressure-bonded material 130 was calculated. TABLE 1 shows themeasurement results.

TABLE 1 PRESSURE- BONDING LOAD P Rα1 Rα2 EVALUA- (×10² N/mm) (%) (%)TION PRESSURE-BONDING 4.0 20.0 80.0 X CONDITION 1 PRESSURE-BONDING 4.220.2 80.8 X CONDITION 2 PRESSURE-BONDING 4.3 20.8 83.2 X CONDITION 3PRESSURE-BONDING 4.6 22.0 88.0 ◯ CONDITION 4 PRESSURE-BONDING 4.8 22.188.4 ◯ CONDITION 5 PRESSURE-BONDING 4.9 22.0 88.0 ◯ CONDITION 6PRESSURE-BONDING 5.1 22.2 88.8 ◯ CONDITION 7 PRESSURE-BONDING 5.8 22.489.6 ◯ CONDITION 8 PRESSURE-BONDING 6.0 24.3 97.2 ◯ CONDITION 9PRESSURE-BONDING 6.7 22.9 91.6 ◯ CONDITION 10 PRESSURE-BONDING 7.1 24.096.0 ◯ CONDITION 11 PRESSURE-BONDING 7.3 24.1 96.4 ◯ CONDITION 12PRESSURE-BONDING 7.8 22.7 90.8 ◯ CONDITION 13

FIGS. 7 and 8 show a portion of a cross-sectional photograph of thepressure-bonded material under the pressure-bonding condition 3 and aportion of a cross-sectional photograph of the pressure-bonded materialunder the pressure-bonding condition 13, respectively. In FIGS. 7 and 8, three different portions of the same rolled material are stacked inthe stacking direction.

(Results of First Example)

From the results shown in TABLE 1, as the pressure-bonding load Pincreased, both Rα1 (the thickness ratio of the minimum thickness tαminto the thickness t11 of the pressure-bonded material 130) and Rα2 (thethickness ratio of the minimum thickness tαmin to the average thicknesstαavg) tended to increase. In the pressure-bonded material subjected toroll-bonding with a pressure-bonding load P of 4.3×10³ N/mm or less(less than 4.4×10³ N/mm) under the pressure-bonding conditions 1 to 3,Rα1 was 20.8% or less (21% or less), and Rα2 was 83.2% or less (lessthan 84%).

Evaluation criteria in TABLE 1 are determined based on whether or notthe ratio of the minimum thicknesses to the average thicknesses of theSUS layers in a final product is 70% or more. For example, in a cladmaterial having a thickness of 1 mm in which a layer thickness ratio ofa SUS layer, a Cu layer, and a SUS layer is 1:2:1, the averagethicknesses of the SUS layer 31 and the SUS layer 33 are 0.25 mm. Whenthe ratio of the minimum thicknesses to the average thicknesses of theSUS layers in the final product of the clad material was 70% (0.175 mm),no crack was generated. However, when the ratio of the minimumthicknesses to the average thicknesses of the SUS layers in the finalproduct of the clad material was 60% (0.150 mm), cracks were sometimesgenerated. This is conceivably because the SUS layers having higherstrength and being less likely to extend than the Cu layer wereexcessively pulled in the rolling direction (longitudinal direction)during rolling. Furthermore, there is a concern that the thickness ofthe clad material and the thickness of each layer constituting the cladmaterial may vary due to practical variations such as materialcharacteristics and rolling. From such a viewpoint, the evaluationcriteria in TABLE 1 was determined based on whether or not the thicknessratio Rβ2 was 70% or more at which no crack was generated.

The thickness ratios Rα1 shown in TABLE 1 were obtained by defining thethickness ratios Rα2 from another viewpoint (the minimum thicknessestαmin with respect to the entire clad material). The thickness ratiosRα2 are thickness ratios of the minimum thicknesses tαmin to the averagethicknesses tαavg of the SUS plates immediately after the clad rolling.In other words, the thickness ratios Rα1 and Rα2 shown in TABLE 1 areratios at the stage during production of the final product. In the stageduring production of the final product and the final product, the valuesof the thicknesses of the entire clad materials and the thicknesses ofthe respective layers are different, but theoretically, the ratios ofthe thicknesses of the respective layers to the thicknesses of theentire clad materials do not change. Thus, the ratios of the minimumthicknesses to the average thicknesses do not change due to furtherrolling. However, in practice, variations occur in the rollingoperation, and thus even when Rα2 at the stage during production of thefinal product is 70% or more, the evaluation of the clad materials inwhich the ratios of the minimum thicknesses to the average thicknessesof the SUS layers of the final product are less than 70% is x(unsuitable). When Rα2 is 85% or more, the ratios of the minimumthicknesses to the average thicknesses of the SUS layers of the finalproduct are highly likely to be 70% or more 70% by performing furtherrolling (intermediate rolling and finish rolling) after the cladrolling, and thus the evaluation of the clad materials is ∘ (suitable).

In the case of the pressure-bonded materials under the pressure-bondingconditions 1 to 3, the variations in the minimum thicknesses are furtherincreased by performing further rolling (intermediate rolling and finishrolling) after the clad rolling, and the minimum thicknesses of firstlayers and the minimum thicknesses of third layers in the clad materialsas final products are conceivably less than 70% of the averagethicknesses of the first layers and the average thicknesses of the thirdlayers, respectively. Therefore, the pressure-bonding conditions 1 to 3are marked with x (unsuitable) as their evaluation.

On the other hand, in the case of the pressure-bonded materialssubjected to roll-bonding with a pressure-bonding load P of 4.4×10³ N/mmor more under the pressure-bonding conditions 4 to 13, Rα1 became 22.0%or more (more than 20.8% and 21% or more), and Rα2 became 88.0% or more(more than 83.2% and 84% or more). This is conceivably because eachlayer was significantly reduced or prevented from being rolled in such amanner as to plastically deform non-uniformly due to a sufficientpressing load P of 4.4×10³ N/mm or more. Even when further rolling(intermediate rolling and finish rolling) is performed on thepressure-bonded materials under the pressure-bonding conditions 4 to 13after the clad rolling such that the variations in the minimumthicknesses are further increased, the minimum thicknesses of the firstlayers and the minimum thicknesses of the third layers in the cladmaterials as final products are conceivably 70% or more of the averagethicknesses of the first layers and the average thicknesses of the thirdlayers, respectively. Therefore, the pressure-bonding conditions 4 to 13are marked with ∘ (suitable) as their evaluation.

As shown in TABLE 1, the values of the thickness ratio Rα1 and thethickness ratio Rα2 of the pressure-bonded material subjected toroll-bonding with a pressure-bonding load P of 6.7×10³ N/mm or moreunder the pressure-bonding condition 10 were smaller than the values ofRα1 and Rα2 of the pressure-bonded material subjected to roll-bondingwith a pressure-bonding load P of 6.0×10³ N/mm or more under thepressure-bonding condition 9. In addition, the values of Rα1 and Rα2 ofthe pressure-bonded material subjected to roll-bonding with apressure-bonding load P of 7.8×10³ N/mm or more under thepressure-bonding condition 13 were smaller than the values of thethickness ratio Rα1 and the thickness ratio Rα2 of the pressure-bondedmaterial subjected to roll-bonding with a pressure-bonding load P of7.3×10³ N/mm or more under the pressure-bonding condition 12. Theseresults are considered as follows.

A force P0 applied to the work rollers 103 a is represented by theproduct of the pressure-bonding load P and the length L of the rollersurface (see the above formula (1)). Therefore, when the length L of theroller surface is constant as in the first embodiment, thepressure-bonding load P and the force P0 applied to the work rollers 103a are in a proportional relationship. That is, as the pressure-bondingload P increases, the force P0 applied to the work rollers 103 a alsoincreases. From this viewpoint, the value of the thickness h2 of thepressure-bonded material 130 on the exit side of the roller 103 varieddue to a variation in the force P0, and Rα1 and Rα2 conceivably happenedto decrease under that influence.

When the thickness h2 of the pressure-bonded material 130 on the exitside of the roller 103 decreases, the plastic deformation ratio of eachlayer increases. Accordingly, it is difficult to obtain uniformity ofthe thickness h2 of the pressure-bonded material 130 on the exit side ofthe roller 103, and it is also difficult to obtain uniformity of thethickness of each layer. Consequently, it is difficult to obtainuniformity of the thickness h2, the thickness ratio Rα1, and thethickness ratio Rα2 of the pressure-bonded material 130 on the exit sideof the roller 103, and the values may vary and may be small or large.However, although it is difficult to obtain uniformity, as can be seenby comparing the result of the pressure-bonding condition 4 with theresult of the pressure-bonding condition 13, for example, thepressure-bonding load is increased such that the thickness ratio Rα1 andthe thickness ratio Rα2 themselves gradually increase.

The tendency of a change of the value of Rα1 of the pressure-bondedmaterial with respect to the pressure-bonding load P shown in TABLE 1can be generally represented by a linear approximation curve having aslope of about 0.87 and an intercept of about 17.5 (a straight linesuitable for data having a simple linear relationship). Similarly, thetendency of a change of the value of Rα2 of the pressure-bonded materialwith respect to the pressure-bonding load P shown in TABLE 1 can begenerally represented by a linear approximation curve having a slope ofabout 3.46 and an intercept of about 69.8. Therefore, there is apositive correlation between the pressure-bonding load P and the valuesof Rα1 and Rα2 of the pressure-bonded material, and it can be confirmedthat as the pressure-bonding load P increases, the values of Rα1 and Rα2of the pressure-bonded material increase.

When P=4.3 (×10³ N/mm) is substituted into a deformation approximationcurve of Rα2, Rα2=84.678(%). When P=4.4 (×10³ N/mm) is substituted,Rα2=85.024(%). When Rα2 is 85% or more as described above, the ratio ofthe minimum thickness to the average thickness of the SUS layer of thefinal product is highly likely to be 70% or more, and thus it isreasonable to set the lower limit of the pressure-bonding load P to4.4×10³ N/mm or more that results in Rα2=85.024(%).

From the cross-sectional photograph shown in FIG. 7 , it has beenclearly confirmable that the interface is formed in a wave shape in thepressure-bonded material under the pressure-bonding condition 3 in whichthe roll-bonding has been performed with a pressure-bonding load P of4.3×10³ N/mm. On the other hand, from the cross-sectional photographshown in FIG. 8 , it has been confirmable that formation of theinterface in a wave shape is significantly reduced or prevented in thepressure-bonded material under the pressure-bonding condition 13 inwhich the roll-bonding has been performed with a pressure-bonding load Pof 7.8×10³ N/mm. Therefore, it has been confirmed that the degree (themagnitude of the undulation) of the wave shape of the interface isclearly different between the pressure-bonded material with x(unsuitable) as the evaluation and the pressure-bonded material with ∘(suitable) as the evaluation. As for the degree of the wave shape of theinterface, when the minimum thickness with respect to the averagethickness in the pressure-bonded material is small (the values of thethickness ratios Rα2 and Rβ2 are small), the amplitude of the wave atthe interface increases, and thus it is known that the degree of thewave shape of the interface becomes larger (more wavy). In other words,the degree of the wave shape of the interface is improved by increasingthe pressure-bonding load.

Second Example

In a second example, a clad material 30 as a final product was producedusing the pressure-bonded material 130 under the pressure-bondingcondition 6 (pressure-bonding load P=4.9×10³ N/mm) in the first example.Materials for a SUS plate 131 and a SUS plate 133 and the thicknessesthereof in the pressure-bonded material 130 were the same, and thus thethickness ratios of the thickness (average thickness t2avg) of a SUSlayer 31 and the thickness (average thickness t3avg) of a SUS layer 33to the thickness of the clad material 30 obtained by rolling thepressure-bonded material 130 were substantially the same (25%). In thiscase, the SUS layers (SUS layers 31 and 33) can be collectivelyevaluated without distinguishing the SUS layer 31 from the SUS layer 33in the clad material 30. From such a viewpoint, the thickness (minimumthickness t2min) of a portion having the smallest thickness (thinnestportion) in the SUS layer 31 (first layer) of the clad material 30 andthe thickness (minimum thickness t3min) of a portion having the smallestthickness (thinnest portion) in the SUS layer 33 (third metal plate) ofthe clad material 30 were measured, and the thickness (minimum thicknesstβmin) of the portion having the smallest thickness (thinnest portion)in the SUS layers and the average value (average thickness tβavg) of thethicknesses of the SUS layers were acquired.

Specifically, intermediate rolling was performed using the roller 105 onthe strip-shaped pressure-bonded material 130 under the pressure-bondingcondition 6 (pressure-bonding load P=4.9×10³ N/mm) in the first example.At this time, the rolling was performed such that the thickness t11 ofthe pressure-bonded material 130 after the intermediate rolling was 67%of the thickness t11 of the pressure-bonded material 130 before theintermediate rolling.

Thereafter, diffusion annealing was performed on the pressure-bondedmaterial under a temperature condition of 950° C. Then, in order toadjust the thickness of the pressure-bonded material 130 after thediffusion annealing, finish rolling was performed on the pressure-bondedmaterial. At this time, the rolling was performed such that thethickness t11 of the pressure-bonded material 130 (clad material 30)after the finish rolling was 85% of the thickness t11 of thepressure-bonded material 130 before the finish rolling. Thus, thestrip-shaped clad material 30 of an example of the present invention wasproduced. Fourteen strip-shaped clad materials 30 of the example of thepresent invention were prepared.

Meanwhile, a clad material of a comparative example with respect to theexample of the present invention was produced. Specifically, astrip-shaped Cu plate made of Cu or a Cu alloy and having a sufficientlysoftened structure was prepared. Note that temper rolling was notperformed on the strip-shaped Cu plate.

Then, in addition to the strip-shaped Cu plate, a pair of strip-shapedSUS plates made of the same stainless steel as in the aforementionedexample of the present invention were prepared. The thickness ratio ofthe SUS plate, the Cu plate, and the SUS plate in the comparativeexample was the same as that in the example of the present invention.

Thereafter, similarly to the example of the present invention, in astate in which the SUS plate, the Cu plate, and the SUS plate werestacked in this order, clad rolling was performed by rolling and bondingusing a roller such that a pressure-bonded material was produced. In thecomparative example, unlike the example of the present invention,roll-bonding was performed with a pressure-bonding load P of 4.3×10³N/mm (the pressure-bonding condition 3 in the first example). A change(rolling reduction) in the thickness of the pressure-bonded materialbefore and after the clad rolling in the comparative example was thesame as that in the example of the present invention.

Thereafter, intermediate rolling was performed on the pressure-bondedmaterial, similarly to the example of the present invention. Then,diffusion annealing was performed on the pressure-bonded material undera temperature condition of 950° C. After that, finish rolling wasperformed on the pressure-bonded material, similarly to the example ofthe present invention such that the clad material of the comparativeexample was produced. Fifteen strip-shaped clad materials of thecomparative example were prepared.

Then, in each of the fourteen clad materials of the example of thepresent invention and the fifteen clad materials of the comparativeexample, ten test regions having a range (length) of 15 mm in a rollingdirection were acquired. At this time, five portions were randomlyacquired in the vicinity of each of opposite ends of the strip-shapedpressure-bonded material in the rolling direction. Then, in each of theten test regions, the minimum thickness t2min of the SUS layer 31 andthe minimum thickness t3min of the SUS layer 33 were acquired, the tenminimum thicknesses t2min and the ten minimum thicknesses t3min werecollectively averaged, and the minimum thickness of the SUS layers (SUSlayers 31 and 33) in the clad material 30 was set to tβmin. Therefore,the above minimum thickness tβmin is based on measured values of twentythicknesses obtained from the clad material.

For each of the ten test regions, the thicknesses of five randomlyselected portions in the SUS layer 31 were measured and averaged toacquire the average thickness t2avg of the SUS layer 31. Similarly, foreach of the ten test regions, the thicknesses of five randomly selectedportions in the SUS layer 33 were measured and averaged to acquire theaverage thickness t3avg of the SUS layer 33. Then, the five averagethicknesses t2avg and the five average thicknesses t3avg obtained fromthe ten test regions were collectively averaged to obtain the averagethickness tβavg of the SUS layer (SUS layers 31 and 33) of the cladmaterial 30. Therefore, the above average thickness tβavg is based onmeasured values of a hundred thicknesses obtained from the clad material30.

Next, in each of the fourteen clad materials of the example of thepresent invention and the fifteen clad materials of the comparativeexample, the thickness ratio Rβ1 (=(tβmin/t1)×100(%)) of the aboveminimum thickness tβmin of the SUS layers (SUS layers 31 and 33) to thethickness t1 of the entire clad material was calculated. Furthermore,the thickness ratio Rβ2 (=(tβmin/tβavg)×100(%)) of the minimum thicknesstβmin to the above average thickness tβavg of the SUS layers (SUS layers31 and 33) was calculated. These results are shown by a so-calledboxplot. FIGS. 9 and 10 show a boxplot of the example of the presentinvention and a boxplot of the comparative example, respectively.

A bar graph relating to the frequencies of Rβ1 and Rβ2 in the cladmaterial of the example of the present invention was prepared using twohundred and eighty (=(10 t2mins+10 t3mins)×14 clad materials) measuredvalues of the minimum thicknesses t2min and the minimum thicknessest3min acquired in the example of the present invention. Then, theaverage value Rβ1avg (=(ΣRβ1)/280) and the standard deviation σ(=√{square root over ( )}((Σ(Rβ1−Rβ1avg)²)/280)) of Rβ1 of the exampleof the present invention were calculated. Similarly, a bar graphrelating to the frequencies of Rβ1 and Rβ2 in the clad material of thecomparative example was prepared using three hundred (=(10 t2mins+10t3mins)×15 clad materials) measured values of the minimum thicknessest2min and the minimum thicknesses t3min acquired in the comparativeexample. Then, the average value Rβ1avg (=(ΣRβ1)/300) and the standarddeviation σ (=√{square root over ( )}((Σ(Rβ1−Rβ1avg)²)/300)) of Rβ1 inthe comparative example were calculated. The bar graph of the example ofthe present invention and the bar graph of the comparative example areshown in FIGS. 11 and 12 , respectively.

(Results of Second Example)

In the clad material of the example of the present invention produced byperforming the intermediate rolling and the finish rolling on thepressure-bonded material with a pressure-bonding load P of 4.9×10³ N/mm(4.4×10³ N/mm or more) (the pressure-bonded material under thepressure-bonding condition 6), the average value Rβ1avg of Rβ1 was 21.2%(see FIG. 11 ). Furthermore, as shown in FIG. 9 , it has beenconfirmable that in the clad material, Rβ1 is 17.5% or more and Rβ2 is70.0% or more. Thus, in the clad material of the example of the presentinvention, it has been confirmable that the occurrence of portionshaving an excessively small thickness in the SUS layers (the first layerand the third layer) made of stainless steel is significantly reduced orprevented. Furthermore, it has been confirmable that in each cladmaterial, the first quartile of Rβ1 is 20% or more and the firstquartile of Rβ2 is 80.0% or more. From these, it has been confirmablethat in many clad materials, the occurrence of portions having anexcessively small thickness in the SUS layers can be reliablysignificantly reduced or prevented.

As shown in FIG. 11 , the standard deviation σ of Rβ1 in the cladmaterial of the example of the present invention was 1.0% (1.5% orless), and a variation in Rβ1 was small. Thus, it has been confirmablethat in a product (such as a chassis) produced from the clad material ofthe example of the present invention, the occurrence of variations incharacteristics such as mechanical strength can be significantly reducedor prevented.

On the other hand, in the clad material of the comparative exampleproduced by performing the intermediate rolling and the finish rollingon the pressure-bonded material with a pressure-bonding load P of4.3×10³ N/mm (less than 4.4×10³ N/mm) (the pressure-bonded materialunder the pressure-bonding condition 3), the average value Rβ1avg of Rβ1was 18.8% (see FIG. 12 ). Furthermore, as shown in FIG. 10 , it has beenconfirmable that in the clad material, a part of Rβ1 is less than 17.5%and a part of R$2 is less than 70.0%. Thus, it has been confirmable thatin the clad material of the comparative example, there is a possibilitythat the SUS layers (the first layer and the third layer) made ofstainless steel include portions having excessively small thicknesses.In addition, it has been confirmable that in each clad material, even apart of the first quartile of Rβ1 is less than 17.5% and even a part ofthe first quartile of Rβ2 is less than 70.0%. From these, it has beenconfirmable that in the Cu layers of many clad materials, there is apossibility that the SUS layers include portions having excessivelysmall thicknesses.

As shown in FIG. 12 , the standard deviation σ of Rβ1 in the cladmaterial of the comparative example was 1.7% (a value exceeding 1.5%),and a variation in Rβ1 was large. Thus, it has been confirmable that ina product (such as a chassis) produced from the clad material of thecomparative example, variations in characteristics such as mechanicalstrength are likely to occur.

Here, Rα1 and Rα2 in the pressure-bonded material under thepressure-bonding condition 4 of the first example (produced with apressure-bonding load within the scope of the present invention (claim4)) were respectively the same as Rα1 and Rα2 in the pressure-bondedmaterial under the pressure-bonding condition 6 (produced with apressure-bonding load within the scope of the present invention (claim4)). Therefore, even when Rβ1 and Rβ2 are measured similarly to theaforementioned second example using the pressure-bonded material underthe pressure-bonding condition 4, it can be inferred that results havingsubstantially no difference from Rβ1 and Rβ2 in the measurement of thesecond example performed using the pressure-bonded material under thepressure-bonding condition 6 are obtained. In other words, even when thepressure-bonded material under the pressure-bonding condition 4 is used,it can be inferred that in the clad material as the final product, Rβ1is 17.5% or more, Rβ2 is 70.0% or more, and the standard deviation σ ofRβ1 is 1.0% (1.5% or less).

Furthermore, when Rα1 and Rα2 are large in the pressure-bonded materialafter the clad rolling, the degree of the wave shape of the interface issmaller than when Rα1 and Rα2 are small. Therefore, as described above,even when a variation in the minimum thickness is further increased byperforming further rolling (intermediate rolling and finish rolling)after the clad rolling, it can be inferred that when Rα1 and Rα2 arelarge, Rβ1 and Rβ2 also become large in the clad material as the finalproduct. Furthermore, it can be inferred that the standard deviation σof Rβ1 becomes small due to the small degree of the wave shape of theinterface.

In other words, when the clad material of the final product is producedusing the pressure-bonded materials under the pressure-bondingconditions 5 and 7 to 13 in which the pressure-bonding load P is largerthan that in the pressure-bonding condition 4 and Rα1 and Rα2 are largerthan those in the pressure-bonding condition 4, it can be inferred thatalso in the clad material as the final product, Rβ1 and Rβ2 increase andthe standard deviation σ of Rβ1 decreases. Therefore, even when the cladmaterial of the final product is produced using the pressure-bondedmaterials under the pressure-bonding conditions 5 and 7 to 13, it can beinferred that in the clad material, Rβ1 is 17.5% or more and Rβ2 is70.0% or more, and it can be inferred that the standard deviation σ ofRβ1 is less than 1.7% (probably 1.5% or less).

From the results of the first and second examples, as long as in thecross-sectional view along the stacking direction of the final product,the minimum thickness of the first layer in the stacking direction andthe minimum thickness of the third layer in the stacking direction are70% or more and less than 100% (the thickness ratio Rβ2 is 70% or moreand less than 100%) of the average thickness of the first layer in thestacking direction and the average thickness of the third layer in thestacking direction, respectively, it is possible to significantly reduceor prevent a decrease in welding strength when the clad material iswelded to another member, and to significantly reduce or prevent theoccurrence of a variation in the mechanical strength of the cladmaterial. Consequently, based on the finding that a clad materialsuitable for welding applications, for example, can be obtained, as thepressure-bonding load at the time of clad rolling at which the thicknessratio Rβ2 in the final product is 70% or more and less than 100%, anumerical range from 4.6×10³ N/mm to 7.8×10³ N/mm is obtained. Asdescribed above, the pressure-bonding load is set to 4.4×10³ N/mm suchthat the minimum thickness of the first layer in the stacking directionand the minimum thickness of the third layer in the stacking directionare highly likely to be 70% or more of the average thickness of thefirst layer in the stacking direction and the average thickness of thethird layer in the stacking direction. Thus, also when thepressure-bonding load is set to 4.4×10³ N/mm, a similar effect isconceivably obtained. Therefore, it is reasonable to set the range ofthe pressure-bonding load to 4.4×10³ N/mm or more and 7.8×10³ N/mm orless.

In the first example in which the thickness ratio Rα2 immediately afterthe clad rolling was obtained, the pressure-bonding conditions 1 to 3were marked with x, and the pressure-bonding conditions 4 to 13 weremarked with ∘. In the second example, the thickness ratio Rβ2 after thefinish rolling was obtained for the comparative example of thepressure-bonding condition 3 (pressure-bonding load: 4.3×10³ N/mm) withthe largest thickness ratio Rα2 (the minimum thickness of the SUSLayers) among the conditions marked with x in the first example and theexample of the pressure-bonding condition 6 (pressure-bonding load:4.9×10³ N/mm) with the smallest thickness ratio Rα2 (the minimumthickness of the SUS Layers) among the conditions marked with ∘ in thefirst example. As shown in FIGS. 9 to 12, in the example, the thicknessratios Rβ2 after the finish rolling in all the test materials were 70%or more, while in the comparative example, portions in which thethickness ratio Rβ2 after the finish rolling was less than 70% occurredin all the test materials. From this, in the clad materials producedunder the pressure-bonding conditions 1 and 2 with the thickness ratioRα2 smaller than that of the pressure-bonding condition 3(pressure-bonding load: 4.3×10³ N/mm), it is easy to imagine that thethickness ratio Rβ2 (the minimum thickness of the SUS layers) after thefinish rolling is smaller than that of the pressure-bonding condition 3.Therefore, in TABLE 1, the pressure-bonding condition 3 is x, and thusboth the pressure-bonding conditions 1 and 2 are also x.

Third Example

In a third example, the grain size of a Cu layer was measured for theclad materials of the example of the present invention and thecomparative example in the second example based on a comparison methodof JIS H 0501. Furthermore, a tensile strength test based on JIS Z 2241was performed on the clad materials of the example of the presentinvention and the comparative example such that tensile strength (aforce at break) and 0.2% proof stress (a force when the elongation was0.2%) as mechanical strength and elongation (((length at break−lengthbefore test)/length before test)×100(%)) as workability were measured.As magnetic characteristics, the relative magnetic permeability of theclad materials of the example of the present invention and thecomparative example was measured. TABLE 2 shows the measurement results.

TABLE 2 MAGNETIC MECHANICAL STRENGTH CHARACTERISTICS WORKABILITY TENSILE0.2% PROOF RELATIVE GRAIN SIZE ELONGATION STRENGTH STRESS MAGNETIC (mm)(%) (MPa) (MPa) PERMEABLITY EXAMPLE OF 0.108 13.5 568 551 1.003 PRESENTINVENTION COMPARATIVE >0.250 5.3 560 546 1.003 EXAMPLE(Results of Third Example)

Regarding the workability, the clad material of the example of thepresent invention in which the grain size of the Cu layer was 0.150 mmor less (0.108 mm) showed an elongation of 13.5%, which was a value of10% or more. That is, it can be said that the clad material of theexample of the present invention has sufficient workability(deformability). On the other hand, in the clad material of thecomparative example in which the grain size of the Cu layer exceeded0.250 mm, the elongation was 5.3%, which was a value of less than 10%.That is, the clad material of the comparative example may not havesufficient workability. Consequently, it can be said that the grain sizeof the Cu layer is set to 0.150 mm or less such that sufficientworkability can be imparted to the clad material.

As for the mechanical strength, no significant difference was observedbetween the example of the present invention and the comparative examplein both the tensile strength and the 0.2% proof stress. Thus, it can besaid that the clad material of the example of the present invention canbe used for a structure such as a chassis. Furthermore, the relativemagnetic permeability as the magnetic characteristics did not differgreatly between the example of the present invention and the comparativeexample, and it has been confirmable that each of the examples hardlymagnetizes. Thus, it can be said that other components (electroniccomponents, for example) can be prevented from being adversely affecteddue to magnetization of the chassis when the clad material of theexample of the present invention is used for the chassis that alsoserves as a heat sink, for example.

Modified Examples

The embodiment and examples disclosed this time must be considered asillustrative in all points and not restrictive. The scope of the presentinvention is not shown by the above description of the embodiment andexamples but by the scope of claims for patent, and all modifications(modified examples) within the meaning and range equivalent to the scopeof claims for patent are further included.

For example, while the example in which the clad material 30 has athree-layer structure in which the SUS layer 31 (first layer) made ofstainless steel, the Cu layer 32 (second layer) made of Cu or a Cualloy, and the SUS layer 33 (third layer) made of stainless steel arestacked in this order has been shown in the aforementioned embodiment,the present invention is not restricted to this. In the presentinvention, the clad material may have a four-or-more-layer structure aslong as the same includes the first layer made of stainless steel, thesecond layer made of Cu or a Cu alloy and roll-bonded to the firstlayer, and the third layer made of stainless steel and roll-bonded tothe side of the second layer opposite to the first layer.

While the example in which the temper rolling, the clad rolling, theintermediate rolling, and the finish rolling are performed in order toproduce the clad material 30 has been shown in the aforementionedembodiment, the present invention is not restricted to this. In thepresent invention, in order to produce the clad material, at least theclad rolling may be performed, and the temper rolling, the intermediaterolling, and the finish rolling may not be performed. For example, in aclad material produced by a process in which the intermediate rollingand the finish rolling shown in FIG. 4 are not performed, the thicknessof the pressure-bonded material (pressure-bonded material 130) after theclad rolling is the same as that of the clad material (clad material 30)as a final product. The clad material (clad material 30) as a finalproduct has a thickness level of 1.0 mm or less, 0.5 mm or less, 0.3 mmor less, or even 0.2 mm or less, for example. In order to easily andreliably perform the clad rolling, it is preferable to perform thetemper rolling. In order to reduce a difference in the thickness of theclad material for each product, it is preferable to perform theintermediate rolling and the finish rolling.

While the example in which the clad material 30 is used as the chassis 3of the portable device 100 has been shown in the aforementionedembodiment, the present invention is not restricted to this. In thepresent invention, the clad material may be used for applications otherthan the chassis of the portable device. For example, the clad materialaccording to the present invention may be used for a conductive memberof a battery. The clad material according to the present invention issuitable for applications in which it is necessary to satisfy one orboth of mechanical strength and corrosion resistance and one or both ofconductivity and heat conductivity.

DESCRIPTION OF REFERENCE NUMERALS

-   30: clad material-   31: SUS layer (first layer)-   32: Cu layer (second layer)-   33: SUS layer (third layer)-   131: SUS plate (first metal plate)-   132: Cu plate (second metal plate)-   133: SUS plate (third metal plate)

The invention claimed is:
 1. A clad material comprising: a first layermade of stainless steel; a second layer made of Cu or a Cu alloy androll-bonded to the first layer; and a third layer made of stainlesssteel and roll-bonded to a side of the second layer opposite to thefirst layer; wherein the clad material has an overall thickness of 1 mmor less; and in a cross-sectional view along a stacking direction, aminimum thickness of the first layer in the stacking direction and aminimum thickness of the third layer in the stacking direction are 70%or more and less than 100% of an average thickness of the first layer inthe stacking direction and an average thickness of the third layer inthe stacking direction, respectively, the elongation of the cladmaterial is 8% or more; and a grain size of the second layer is 0.150 mmor less.
 2. The clad material according to claim 1, wherein a standarddeviation of the minimum thickness of the first layer with respect to athickness of the clad material and a standard deviation of the minimumthickness of the third layer with respect to the thickness of the cladmaterial are 1.5% or less.
 3. The clad material according to claim 1,wherein the first layer and the third layer are both made of austeniticstainless steel.